1. Field
The present invention relates to coherent detection of pulse position modulated signals. More particularly, the present invention relates to a top hat optical pulse generator for use within a pulse position modulation receiver.
2. Description of Related Art
Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to address this need is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, a temporal shift in the pulse position represents a sample of the transmitted waveform. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude modulated signal is shown below:SNRppm∞SNRam(tp/τ)2  Eq.(1) where tp is the temporal spacing between unmodulated pulses and τ is the pulse duration, respectively.
Conventional detection or demodulation of analog PPM optical signals, though, suffers from poor SNR at low frequencies. PPM signals are usually demodulated from the optical to electronic domain by a photodiode followed by a lowpass filter (LPF) that converts pulse position modulation to amplitude modulation. Such a demodulation technique is not capable of recovering the DC component, since the DC component is represented by a constant temporal shift of all pulses from their unmodulated positions. Moreover, the demodulated signals after the lowpass filter have very low amplitude at low frequencies. The amplitude increases linearly with frequency up to the Nyquist limit. Such frequency-dependent distortion is corrected by an integration circuit, which amplifies low-frequency noise accordingly, resulting in decreased SNR performance.
An apparatus and method for detecting an optical PPM signal are described in commonly-owned U.S. Pat. No. 6,462,860, issued to lonov on Oct. 8, 2002, incorporated herein by reference. This patent application describes coherent wavelength converters that are used to generate preferably top hat shaped optical pulses. Top hat shaped optical pulses are preferred in the apparatus and method described in U.S. Pat. No. 6,462,860, since such pulses provide for better linearity of the detected optical PPM signal. Top hat shaped optical pulses may also be used in other optical systems known in the art. However, U.S. Pat. No. 6,462,860 describes the generation of top hat shaped pulses with zero dispersion fiber. Zero dispersion fiber is not generally commercially available. Therefore, there exists a need in the art for an optical pulse generator that can generate optical pulses with a top hat shape without requiring the use of zero dispersion fiber.